Whoops. This description is for rubber O-rings, not shower curtain hangers. @ThunderThighs ?
O-rings are one of the simplest, yet most engineered, precise and useful seal designs ever developed. They are available in various metric and in standard sizes. Sizes are specified by the inside diameter and the cross section diameter (thickness)
That’s the description straight from Amazon.
https://www.amazon.com/Homebasix-SD-ORING-W3L-Shower-Curtain-Pieces/dp/B002MS6JIK
So what? It’s wrong.